I was alerted by some vibrating noise in my basement and opened a little access hatch to learn more.
I saw a metal box with electric wires in it. This box is screwed in a larger pipe, which seems to run although the heating/vent is off. The 2 metal elements were vibrating against each other, causing the sound.
It's a junction box for some item installed in the duct (based on the "other" wires appear to go out the back into the duct.) Terminology wise, we tend to reserve "pipe" for a round tubular object with a bit more structure used for carrying water or gasses at pressure, while duct refers to the thin sheetmetal for moving air at barely any pressure around the house. You have a duct there.
That might be a duct booster fan, or a motorized damper, off the top of my head for most likely in-duct electrified objects, though the booster fan is more likely since damper actuators (the powered part) are usually external to the duct.
It’s probably powering a fan, possibly either part of a radon mitigation system or for extra ventilation.
